Pectin–Cellulose Interactions in the <i>Arabidopsis</i> Primary Cell Wall from Two-Dimensional Magic-Angle-Spinning Solid-State Nuclear Magnetic Resonance

The primary cell wall of higher plants consists of a mixture of polysaccharides whose spatial proximities and interactions with each other are not well understood. We recently obtained the first two-dimensional (2D) and three-dimensional high-resolution magic-angle-spinning ¹³C solid-state nuclear magnetic resonance spectra of the uniformly ¹³C-labeled primary cell wall of <i>Arabidopsis thaliana</i>, which allowed us to assign the majority of ¹³C resonances of the three major classes of polysaccharides: cellulose, hemicellulose, and pectins. In this work, we measured the intensity buildup of ¹³C–¹³C cross-peaks in a series of 2D ¹³C correlation spectra to obtain semiquantitative information about the spatial proximities between different polysaccharides. Comparison of 2D spectra measured at different spin diffusion mixing times identified intermolecular pectin–cellulose cross-peaks as well as interior cellulose–surface cellulose cross-peaks. The intensity buildup time constants are only modestly longer for cellulose–pectin cross-peaks than for interior cellulose–surface cellulose cross-peaks, indicating that pectins come into direct contact with the cellulose microfibrils. Approximately 25–50% of the cellulose chains exhibit close contact with pectins. The ¹³C magnetization of the wall polysaccharides is not fully equilibrated by 1.5 s, indicating that pectins and cellulose are not homogeneously mixed on the molecular level. We also assigned the ¹³C signals of cell wall proteins, identifying common residues such as Pro, Hyp, Tyr, and Ala. The chemical shifts indicate significant coil and sheet conformations in these structural proteins. Interestingly, few cross- peaks were observed between the proteins and the polysaccharides. Taken together, these data indicate that the three major types of polysaccharides in the primary wall of <i>Arabidopsis</i> form a single cohesive network, while structural proteins form a relatively separate domain